Modern oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
Among the available wireline logging and LWD tools are a variety of resistivity logging tools including, in particular, laterolog tools. Such tools typically include a central electrode around a tool body, with guard electrodes symmetrically spaced above and below the central electrode. The tool drives auxiliary currents between the guard electrodes and the center electrode to “focus” the current from the center electrode, i.e., to reduce dispersion of the current from the center electrode until after the current has penetrated some distance into the formation. Generally speaking, a greater depth of investigation can be achieved using more widely-spaced guard electrodes, but the vertical resolution of the measurements may suffer. Accordingly, existing tools employ multiple sets of guard electrodes at different spacings from the central electrode to enable multiple depths of investigation without unduly sacrificing vertical resolution.
A problem often experienced with many logging tools, and with laterolog tools in particular, is signal cross-coupling. When signal-carrying conductors are placed in close proximity, they tend to couple capacitively and/or inductively. Such couplings enable signal energy to leak from one conductor to another. Where conductors carrying weak signals (e.g., voltage measurements) receive energy leakage from conductors carrying strong signals (e.g., transmitter currents), this signal cross-coupling can severely impair the tool's performance. The most popular approaches to this problem generally involve heavy shielding and maximal separation of the conductors, but so far only limited successes have been achieved in this manner.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.